Fig. 10. Summary of proposed role for Foxa2, Nodal and Hh signalling pathways in ventral CNS development. The model is based upon data in this paper and in other papers cited in the text. Nodal signalling induces MFP through Madh/Smad and FoxH1/Fast/Sur transcription factors. This occurs in the absence of Foxa2 function, implicating other factors downstream of Nodal in the earliest induction of floorplate identity. Nodal signalling induces foxa2 expression in MFP and Foxa2 function is required for differentiation of these cells. Hh genes are among those requiring Foxa2 function for maintained expression. However, as Hh genes are initially expressed in mol^-/- embryos, we suggest that other transcription factors, including Madh2/FoxH1, contribute to the presence of Hh proteins in the ventral neural tube. The only known role for Hh proteins in the MFP is to maintain expression of differentiation markers. Hh signals working through Gli proteins induce foxa2 expression in the LFP. Foxa2 activity is required for proper formation of the LFP and this is likely to be due both to non-autonomous roles (e.g. regulation of Hh production in the MFP), and activity within the LFP itself. Hh activity also spreads further to induce and pattern adjacent ventral CNS cell types, including cranial motoneurones and serotonergic raphé neurones. In the absence of Foxa2 function, Hh activity leads to ectopic dorsal expression of several Hh pathway target genes. This implies that Foxa2 may also negatively regulate the dorsal spread of Hh activity within the ventral CNS.